(1) Field of the Invention
This invention relates to multisignal amplification, and concerns in particular the use of individual electronic amplification systems for the amplification of a multiplicity of independent electronic signals.
(2) Description of the Prior Art
For many applications it is desirable that a single electronic amplifier be used for the amplification of two or more signals, the individual signals not being connected with or related to each other in any way. A typical such application is in the field of high frequency radio transmission using wideband amplifiers (often of the distributed-amplifier type) feeding radio frequency power to a transmitting aerial for radiation. It is common for two or more quite unconnected signals to be fed to a single amplifier at the same time; an example is the use of a single such amplifier to be employed simultaneously to amplify, for transmission, two or more information-modulated carrier wave signals based upon carrier waves of quite different frequencies.
Amplification of any signal of the continuous wave type involves the generation of harmonics--that is, spurious signals of frequencies which are whole number multiples of the basic signal frequency (thus, twice, three times, four times and so on). For a single basic frequency these harmonics are reduced in power level to the point at which--in general--they can be ignored. However, the "combined" amplification of a number of signals of quite different basic frequencies inevitably gives rise not only to harmonics but also to the highly undesirable formation of what are known as "intermodulation products"--that is, extra spurious output signals related in some way to a combination of the basic input signals and their harmonics. By way of example, in the simple case where a signal of frequency F.sub.1 is being amplified simultaneously with a signal of frequency F.sub.2, there will be generated not only the harmonics 2F.sub.1, 3F.sub.1, 4F.sub.1 . . . and 2F.sub.2, 3F.sub.2, 4F.sub.2 . . . but also intermodulation products of the type F.sub.1 .+-.F.sub.2, 2F.sub.1 .+-.F.sub.2, 3F.sub.1 .+-.2F.sub.2, . . . and so on. The problem is that there may be so many of these intermodulation product signals that they cause unacceptable interference to other signals over a very wide portion of the available radio spectrum.
The situation becomes very much worse as the number of independent basic signal frequencies increases. The number of combinations increases very rapidly as the number of signals increases; the number of possible significant intermodulation combinations for just six basic frequencies can be over a quarter of a million!
One type of amplifier commonly used as a wideband amplifier is that known as a distributed amplifier. For the purpose of this Specification a distributed amplifier may be defined as an amplifier wherein the parasitic capacitance(s) associated with the electrodes (or like operating structures) of the or each amplifying element in the amplifier is/are utilised to form part of input and output transmission lines across which the or each amplifying element is connected. Most commonly, a practical distributed amplifier has two or more individual amplifying elements (perhaps as many as eight), and such a practical distributed amplifier may be further defined as an amplifier in which a number of individual amplifying elements are arranged in parallel to take a control signal from one common input line and to feed an amplified signal to one common output line, the arrangement being such that the control signal arrives sequentially at each amplifier element, and such that all the individual outputs add in phase to give the maximum output. Each amplifying element thus contributes a part of the final output, which final output is effectively merely the sum of the individual parts.
Distributed amplifiers are well known, and various types used as wideband amplifiers are described in, for example, Marconi British Patent Specification No. 846,633 (corresponding to U.S. Pat. No. 3,129,387), and, more recently, in the Specification of Marconi Application for British Letters Patent No. 44,336/78 (corresponding to U.S. application Ser. No. 88,063 filed Oct. 24, 1979, and now U.S. Pat. No. 4,337,439). A basic distributed amplifier format is described hereinafter with reference to the schematic circuit diagram of FIG. 1 of the accompanying drawings.